The present invention relates to a piezoelectric substrate which is used for a surface acoustic wave device having interdigitated electrodes formed on the piezoelectric substrate, and to the surface acoustic wave device.
In recent years, mobile communication terminals such as cellular phones have rapidly become widespread. Such terminals are desired to be reduced in size and weight from the viewpoint of portability. In order to achieve reduction in size and weight of the terminals, electronic parts used therein are required to be reduced in size and weight, too. For that purpose, surface acoustic wave devices, i.e., surface acoustic wave filters, which are advantageous for reduction in size and weight, are often employed in high- and intermediate-frequency portions of the terminals. Such surface acoustic wave devices are formed with interdigitated electrodes on a piezoelectric substrate, for the purpose of exciting, receiving, reflecting or propagating surface acoustic waves.
Important characteristics of a piezoelectric substrate used for a surface acoustic wave device include the surface wave velocity of the surface acoustic wave (hereinafter referred to as SAW velocity), the temperature coefficient of a center frequency in the case of filters or of a resonant frequency in the case of resonators (hereinafter referred to as the temperature coefficient of frequency), and k2, i.e., the square of the electromechanical coupling coefficient (hereinafter k2 is also referred to as the electromechanical coupling coefficient).
FIG. 27 illustrates the types and characteristics of the substrates which have been conventionally used as a piezoelectric substrate for a surface acoustic wave device so far. Hereinafter, these piezoelectric substrates are distinguished from each other using the symbols in FIG. 27.
As can be seen from FIG. 27, the conventionally used piezoelectric substrates are broadly classifiable into two groups: one including 128LN, 64LN, and 36LT that have a high SAW velocity and a large electromechanical coupling coefficient, and the other including LT112 and ST quartz that have a relatively low SAW velocity and a small electromechanical coupling coefficient. The piezoelectric substrates having a high SAW velocity and a large electromechanical coupling coefficient (128LN, 64LN, and 36LT) are used for surface acoustic wave filters in high-frequency portions of terminal devices, for example. On the other hand, the piezoelectric substrates having a relatively low SAW velocity and a small electromechanical coupling coefficient (LT112 and ST quartz) are used for surface acoustic wave filters in intermediate-frequency portions of terminal devices, for example. The reasons for this are as follows.
That is, for a surface acoustic wave filter, its center frequency is substantially proportional to the SAW velocity of the piezoelectric substrate used therein and substantially inversely proportional to the width of an electrode digit of the interdigitated electrodes formed on the substrate. For this reason, to form filters to be used for high-frequency circuit portions, it is preferable to use a substrate having a high SAW velocity. In addition to this, since a wide passband width of 20 MHz or more is required for the filters used for high-frequency portions of terminal devices, it is also necessary for the filters to have a large electromechanical coupling coefficient.
On the other hand, a frequency band ranging from 70 to 300 MHz is used as the intermediate frequency of mobile terminals. When a filter having a center frequency within this frequency band is formed by using a surface acoustic wave device, if a substrate having a high SAW velocity is used as the piezoelectric substrate, it would be necessary to significantly increase the width of an electrode digit formed on the substrate in response to the decreased amount of the center frequency, as compared with that of a filter used for a high-frequency circuit portion. This may present a problem that the surface acoustic wave device itself is increased in size.
For that reason, LT112 and ST quartz having a low SAW velocity are used as the piezoelectric substrates for intermediate-frequency surface acoustic wave filters. In particular, ST quartz is preferable because its first order temperature coefficient of frequency is zero. ST quartz can only form a filter having a narrow passband, due to its small electromechanical coupling coefficient. However, since the function of intermediate-frequency filters is to allow only signals of one narrow channel to pass through, the small electromechanical coupling coefficient has not presented a serious problem.
However, in recent years, from the viewpoints of effective use of frequency resources, compatibility to digital data communications and so on, a digital mobile communication system has been developed and put to practical use to rapidly become widespread. The passband width of this system is very wide, that is, several hundred kHz to several MHz. In the case where an intermediate-frequency filter having such a wide passband is formed using a surface acoustic wave filter, it is difficult to implement it with an ST quartz substrate that has a small electromechanical coupling coefficient.
On the other hand, to enhance size reductions of mobile terminals to improve its portability, it is necessary to reduce the mounting area of the intermediate-frequency surface acoustic wave filter. However, both ST quartz and LT112, which are considered to be suitable for the intermediate-frequency surface acoustic wave filter, have a SAW velocity higher than 3000 m/s (second), which poses limitations in size reductions thereof.
It is an object of the present invention to provide a piezoelectric substrate, for use in a surface acoustic wave device, having a large electromechanical coupling coefficient that is effective to achieve a wider passband and having a low SAW velocity that is effective to reduce the size of s surface acoustic wave device, and to provide a surface acoustic wave device which can achieve a wider passband and size reductions.
A piezoelectric substrate for a surface acoustic wave device according to the invention is intended for use for a surface acoustic wave device having an interdigitated electrode formed on the piezoelectric substrate, and is composed of a single crystal belonging to the point group 32, having a crystal structure of Ca3Ga2Ge4O14 type, containing Ca, Nb, Ga, Si and O as main components, and being represented by the chemical formula Ca3NbGa3Si2O14.
According to the piezoelectric substrate for a surface acoustic wave device of the invention, it is possible to obtain a large electromechanical coupling coefficient that is effective to achieve a wider passband and a low SAW velocity that is effective to reduce the size of a surface acoustic wave device.
A surface acoustic wave device according to the invention has an interdigitated electrode formed on a piezoelectric substrate, the piezoelectric substrate being composed of a single crystal belonging to the point group 32, having a crystal structure of Ca3Ga2Ge4O14 type, containing Ca, Nb, Ga, Si and O as main components, and being represented by the chemical formula Ca3NbGa3Si2O14.
The surface acoustic wave device of the invention allows use of a piezoelectric substrate having a large electromechanical coupling coefficient that is effective to achieve a wider passband and a low SAW velocity that is effective to reduce the size of a surface acoustic wave device. As a result, it is possible to widen the passband and reduce the size of the surface acoustic wave device.
In the surface acoustic wave device or the piezoelectric substrate for the surface acoustic wave device of the invention, when a cut angle at which the piezoelectric substrate is cut out of the single crystal and a propagation direction of a surface acoustic wave are represented in terms of Eulerian angles (xcfx86, xcex8, "psgr"), xcfx86, xcex8, and "psgr" may be in any of the following areas.
[Area 1-1]
xcfx86=xe2x88x925xc2x0 to 5xc2x0,
xcex8=30xc2x0 to 90xc2x0, and
"psgr"=0xc2x0 to 75xc2x0
[Area 1-2]
xcfx86=xe2x88x925xc2x0 to 5xc2x0,
xcex8=110xc2x0 to 155xc2x0, and
"psgr"=60xc2x0 to 80xc2x0
[Area 2-1]
xcfx86=5xc2x0 to 10xc2x0 (excluding 5xc2x0),
xcex8=30xc2x0 to 60xc2x0, and
"psgr"=xe2x88x9275xc2x0 to xe2x88x9230xc2x0
[Area 2-2]
xcfx86=5xc2x0 to 10xc2x0 (excluding 5xc2x0),
xcex8=110xc2x0 to 155xc2x0, and
"psgr"=xe2x88x9285xc2x0 to xe2x88x9265xc2x0
[Area 3-1]
xcfx86=10xc2x0 to 20xc2x0 (excluding 10xc2x0),
xcex8=30xc2x0 to 60xc2x0, and
"psgr"=xe2x88x9275xc2x0 to xe2x88x9230xc2x0
[Area 3-2]
xcfx86=10xc2x0 to 20xc2x0 (excluding 10xc2x0),
xcex8=110xc2x0 to 155xc2x0, and
"psgr"=xe2x88x9285xc2x0 to xe2x88x9265xc2x0
[Area 4-1]
xcfx86=20xc2x0 to 30xc2x0 (excluding 20xc2x0),
xcex8=30xc2x0 to 60xc2x0, and
"psgr"=xe2x88x9280xc2x0 to xe2x88x9240xc2x0
[Area 4-2]
xcfx86=20xc2x0 to 30xc2x0 (excluding 20xc2x0),
xcex8=110xc2x0 to 155xc2x0, and
"psgr"=xe2x88x9275xc2x0 to xe2x88x9255xc2x0
[Area 5]
xcfx86=5xc2x0 to 30xc2x0 (excluding 5xc2x0),
xcex8=30xc2x0 to 90xc2x0, and
"psgr"=xe2x88x9230xc2x0 to 30xc2x0
Other objects, features and advantages of the invention will be sufficiently apparent from the following description.